This is the default behavior: if the attribute on which you add validation feedback is not visible on the current page, Mendix displays the message in a pop-up. In order of workable solutions, you should either 1) accept this default behavior, 2) validate when the attribute is on the page, 3) skip the validation when the attribute is not on the page, 4) try to create your own way to display feedback, e.g. using a non-persistent object on your page to gather all messages.